Celebrity biographer Andrew Morton, who launched his career with a controversial 1992 book on Princess Diana (“Diana: Her True Story — In Her Own Words’’), is already preparing for the day Meghan Markle marries Prince Harry in May.

“I’ve got my top hat being polished,” he jokes.

Morton’s biography of Markle, the American actress and editor, will be published shortly before the wedding. In the meantime, he’s hitting the road to talk about his just-released book, “Wallis in Love,” which focuses on the life of the domineering late duchess of Windsor Wallis Simpson — with key research coming from Boston Public Library archives.

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While researching his last book, “17 Carnations: The Windsors, the Nazis, and the Cover-Up,” Morton became intrigued by Herman Rogers, a well-to-do Yalie and longtime Simpson friend.

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His research took him from a diary in a Buenos Aires bank vault to the Massachusetts Historical Society, where he uncovered correspondence between Simpson and high-living Boston brahmin Constance Coolidge. “Her life was dripping with sex and longing and lust,” Morton says. “She’s worthy of a biography on her own.”

Next he set up shop at the City of Boston Archival Center in West Roxbury, which holds the papers of the late Cleveland Amory. Amory, the writer and animal-rights advocate, had once agreed to ghost write Simpson’s memoir but got cold feet when he realized his subject’s utter lack of interest in the literary arts.

“She gleefully admitted she’d never read a book in her life,” Morton says. “He was horrified.”

Digging through Amory’s 166-box archive Morton came across Amory’s project notes and transcripts of interviews with members of Simpson’s inner circle.

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“There was one very cryptic exchange between Cleveland and Herman,” Morton says. In it, Rogers intimated that Wallis said she loved him and that if she got pregnant just before the wedding, the baby would be seen as Edward’s. For Morton, this was proof that Simpson
loved another man — even though Edward had give up the throne to marry her.

“The groundwork had been laid before; let’s face it,” he says. “Anybody who’s spent 10 minutes with this story knows it wasn’t the great romance of the century.’’

Morton will discuss his book on Feb. 21, from 6-7:30 p.m. at the BPL’s main branch, 200 Boylston St.